1. Field of the Invention
The present invention relates to an image input device for two-dimensionally scanning a planar object (hereinafter referred to as the “document”) such as, for example, a document, photograph and the like, and reading the image data thereby, and an image input/output system using the image input device.
2. Description of the Related Art
Drum type scanners and flatbed type scanners are known as conventional image input devices (scanners) capable of reading the image data of documents at high resolution.
In the case of drum scanners, a document is adhered to the inside of a transparent hollow drum formed, for example, of glass or the like, and the drum is rotated about a center axis at constant speed (main scan). Furthermore, a point sensor is moved parallel to the center axis of the drum from the exterior side of the drum (subscan) to obtain the image data of all regions of the document.
In the case of flatbed scanners, a document is placed face down on the surface of a transparent document table formed of, for example, glass, and a line sensor integrated with an illumination device moves in a direction (subscan) perpendicular to the array direction (main scan direction) of photoelectric conversion elements of the line sensor so as to obtain the image data of the entire region of the document.
In the case of a drum scanner, it is possible to increase the number of image sensings and increase the resolution by slowing both the speed in a first main scan direction (drum rotation speed) and the speed in the subscan direction (sensor moving speed), or shortening the cycle of the sensor load accumulation time and the load transfer time. However, the former method lengthens time required for image sensing of a single document, thus slowing the image sensing speed, On the other hand, the latter method shortens the load accumulation time of each image sensing and reduces S/N due to the small load accumulated by each photoelectric conversion element. Although it is possible to increase S/N by increasing the brightness of the illumination device, a disadvantage arises in that energy consumption also increases. Furthermore, since a large drive device is required due to rotate the heavy drum at high speed, another disadvantage arises in that a great deal of energy is consumed.
In the case of a flatbed scanner, the resolution in the main scan direction is determined by the number of pixels of the line sensor, and the resolution cannot be changed. On the other hand, it is possible to increase the number of image sensing to improve the resolution by reducing the speed (sensor moving speed) in the subscan direction, or shortening the cycle of the sensor load accumulation time and load transfer time so as to increase the resolution in the subscan direction. However, in the case of a drum scanner, the time required to sense one image is lengthened, and the image sensing speed is slowed, similar to the case of a drum scanner. In the latter case, on the other hand, the load accumulation time is shortened such that the S/N is reduced due to the light load accumulation on each photoelectric conversion element. Although it is possible to increase the S/N by increasing the brightness of the illumination device, a disadvantage arises in that energy consumption also increases. Furthermore, since a large drive device is required due to rotate the heavy drum at high speed, another disadvantage arises in that a great deal of energy is consumed.
In both drum scanners and flatbed scanners, the main scan direction and the subscan direction are determined by the mechanical construction, and may not necessarily match the main scan direction and the subscan direction of the image data output device, e.g., a display device such as a CRT, or a printer such as a laser printer. Since the main scan direction of the scanner and the main scan direction of the output device may not match, disadvantages arise inasmuch as the transmission efficiency of the obtained image data is lowered, and high speed processing is not possible.